How to texture paint low poly model with normal applied
I was facing exactly the same problem as you and fortunately I found the solution.
What you need to do is to apply the texture you're painting to the low poly model.
To apply the texture to the model, just go to the shading tab and create a 'Image Texture' node. Select the texture you're painting and finally plug the color to the base color of Principal Node.
In the end, you will get something like this:

Don't forget to select 'lookdev' mode in order you can see the texture and the normal map while painting.